The present invention relates to a method for fabricating a semiconductor device which comprises a step of forming a semiconductor region in a semiconductor substrate by ion implantation.
A thermal diffusion process has been known as one of the methods for doping a p-type or n-type impurity into a semiconductor substrate which is composed of monocrystalline silicon, monocrystalline germanium or the like. In this diffusion process, an impurity at high temperature is diffused into the surface of the semiconductor substrate. However the method of ion implantation has been developed recently and has found wider applications. In ion implantation, impurity ions are accelerated directly at a high voltage, and are implanted into a semiconductor substrate from the surface thereof. This method allows control of the distribution of the impurity more easily than the thermal diffusion process, so that the impurity can be implanted at a higher concentration. In addition, those impurities which cannot be diffused by thermal diffusion can be implanted, which is another advantage of the ion implantation method. Therefore, ion implantation is used as a method for fabricating a semiconductor device of high precision without variation. Annealing is performed in this method to correct crystal defects produced during the ion implantation of the impurity as well as to activate the implanted ions. However, this annealing has the following difficulties: if ions are implanted at high concentration and the ion-implanted region has become uncrystallized, the activation of the impurity ions and the recrystallization of the crystals are achieved by the annealing, but small local segregation of impurity ions are formed. Alternatively, if ions are implanted at a relatively low concentration and the ion-implanted region is not uncrystallized, the impurity ions are activated by the annealing, but the crystals are not well reoriented. Therefore, ion tracks which are distortions of the crystal structure which the impurity ions have disoriented traverse the region until they are stopped in the crystals. In either case, there remain a number of crystal defects in the ion-implanted region. This significantly reduces the lifetime of the carriers produced by the ion implantation and damages the function of the semiconductor device.